The invention relates in general to the field of nanoscale optical sensing devices and, in particular, to optical devices allowing Raman spectroscopy, conducted on very small quantities of analytes in field-enhanced volumes, through optical antennas.
Direct optical sensing of molecular compounds requires chemical bond-specific signatures to unambiguously detect and identify the analytes under test. Commonly employed gas/liquid chromatography combined with mass spectroscopy allows molecular compounds to be identified due to their distinct mass-charge ratio. However, such a method does not make it possible to retrieve chemical information, namely the chemical bonding properties of the atoms, from the sample. Electrical vibration spectroscopy (e.g., Inelastic Electron Tunneling Spectroscopy) directly reveals the chemical binding energies of a molecular compound but with low energy resolution (e.g., 5 meV/˜40 cm−1) and, this, only in a very narrow energy range (up to a few 100 mV/˜800 cm−1).
Besides chromatography-mass spectrometry, fluorescence measurements typically require the attachment of fluorescent labels to the analyte, because the inherent fluorescence of the analytes is usually too weak or not present at specific wavelengths. Such a labelling technique is invasive, can interfere with parameters inherent to the molecules under study and is sometimes not applicable if the native analytes have to be detected without previous labeling.
Optical methods based on elastic or inelastic light scattering at molecular bonds such as absorption or Raman spectroscopy provide sufficient resolution (<0.1 meV/<1 cm−1) to unambiguously identify and differentiate chemical bonds over an extended energy range (˜500 meV/˜4 000 cm−1), thereby enabling a comprehensive analysis. While Raman spectroscopy is non-invasive, its major drawback, however, is that it suffers from a very low optical scattering cross-section, i.e., on the order of 10−30 to 10−31 cm2/sr, that is, about 1 000 times smaller than the cross section for elastic Rayleigh light scattering or absorption. To overcome the low interaction mechanism, millions of identical molecules need typically be probed simultaneously, in multi-path geometries or during long integration times to obtain detectable signal levels. This limitation has, so far, prevented the use of Raman spectroscopy for detection of small volumes, low analyte concentrations and on fast time scales.